The present invention relates to a mixing apparatus.
The operation of mixing is generally understood to comprise two distinct actions; dispersive mixing and distributive mixing. In dispersive mixing the individual parts of the materials being mixed, whether solid or fluid, have their respective geometries altered by means of applied stresses. This usually takes the form of reducing the average size of individual parts while increasing their numbers. In distributive mixing the individual parts of the materials, whether solid or fluid, are blended together in order to obtain a spatial uniformity in the distribution of the various material parts with respect to one another. A good mixing operation thus usually requires both dispersive and distributive mixing actions to occur.
Distributive mixing is primarily a function of the geometry of the mixing apparatus and known mixers typically fall into two general types providing either random or structured distributive mixing. Random distributive mixers achieve mixing by randomly agitating the materials and include known mixers such as tumbleblenders and ribbon-blenders. Structured-distributive mixers on the other hand achieve mixing by systematically repeating a geometrically controlled sequence of dividing, reorienting and rejoining the materials and include static mixers and cavity transfer mixers.
In contrast, dispersive mixing is primarily a function of forces, pressures, stresses and stains applied to the materials. In general, the size reduction of materials that is required in dispersive mixing is achieved by applying stresses to the materials. These applied stresses usually take the form of compressive, tensile or shear stresses. For mixing fluid materials the predominant method of stressing has been by means of applying shear, as this can readily be achieved by utilizing the drag forces that exist within a fluid bounded by two relatively moving surfaces in machine. Examples of such mixers include internal rotor/stator mixers in which the material is sheared between the rotor and the stator surfaces. Shear stressing can also be obtained by forcing a fluid material over one or more surfaces that do not have a motion relative to one another, for instance between the walls of a channel. In this case it is still possible to generate significant shear stresses in the fluid, but only at the expense of proving some form of pumping energy to propel the fluid over the surfaces. It has long been recognized however that an alternative mechanism, that of extensional flow, is capable of subjecting fluid materials to compressive and tensile stresses that in practice can be much higher than the sheer stresses.
Extensional flow requires that the fluid be pressurized in order to propel it between surfaces that subject the fluid to tensile or compressive stresses. Such surfaces can be generally orientated in the direction of the flow in which case the flowing material is accelerated or decelerated along its flow-path by virtue of mass conservation, or generally orientated across the direction of the flow, in which case the flowing material is decelerated and thus compressed by virtue of the change in the momentum of the fluid, such as in impact. Known mixers designed to operate on the basis of extensional flows for dispersion have thus required external means of pressurization in the form of high-pressure pumps located upstream (the same requirement for pumping applies to a mixer operating on the basis of shear flow between non-moving surfaces as mentioned above). Given that it is often a requirement that any given part of the material being mixed is subjected to a number of stressing cycles it is apparent that the overall pressures required to provide extensional flows and shear flows through a mixer can become prohibitively high. Additionally, the need to engineer such a mixer so as to ensure that the extensional flow and shear flow occur with maximum efficiency, i.e. the minimum pressure loss, is relatively costly.
It is an object of the present invention to provide an apparatus which obviates or mitigates the above disadvantages.
According to the present invention there is provided apparatus for mixing a material contained in a vessel, the apparatus comprising one or more flow channels and at least one pair of first and second members which are coaxially mounted one within the other about an axis of rotation such that facing surfaces of the first and second members curve around said axis defining a chamber therebetween, at least one or other of the first and second members having a non-circular curvature, the first and second members being rotatable relative to one another about said axis to thereby produce a pumping force to force material through said flow channels and chamber, wherein the radially outermost of said first and second members and a wall of the vessel move relatively to one another to stress and displace material present between the mixing apparatus and the wall of the vessel.
It is to be understood that the term xe2x80x9ccurvaturexe2x80x9d is not limited to a continually curving surface and thus each of the surfaces may have substantially linear portions.
Apparatus in accordance with the present invention provides that the radial spacing between the first and the second members changes about the axis of rotation as the two rotate relative to one another. An important aspect of the invention is that this effect is achieved with coaxially mounted-members. For instance, a similar effect is achieved by prior art mixers incorporating eccentrically mounted rotors and/or stators. The arrangement in accordance with the present invention is advantageous in that is dynamically more stable as it provides substantial pressure equalization and balance radially and circumferentially across the relatively rotatable members.
The relative movement between the mixing apparatus and the vessel (such as by rotation of the outermost one of said first and second relatively rotatable members) provides for a direct interaction between the mixing apparatus and the wall of the vessel so that a mixing action occurs between the vessel wall and the mixing apparatus. This mixing action may form a fundamental part of the overall mixing action, particularly where the size of the vessel is closely matched to the size of the mixing apparatus.